This invention relates generally to imaging systems which may be used, for example, in connection with digital cameras, scanners, and the like.
Imaging sensors based on silicon technology typically use an infrared blocking element in the optical chain. The purpose of this infrared blocking element is to prevent infrared radiation from entering the imaging array.
Silicon-based devices are typically sensitive to light with wavelengths up to about 1200 nm. If the infrared is permitted to enter the array, the array responds to the infrared, and generates an output image signal. Since one purpose of an imaging system is to create a representation of visible light, the infrared produces a false response is and distorts the image produced by the imaging system. In monochrome (black and white) imaging systems, the result can be an obviously distorted rendition. For example, foliage and human skin tones may appear unusually light. In a color imaging system, the introduction of infrared distorts the coloration and produces an image with incorrect color.
Liquid crystal color filters may be used to produce a switchable shutter made up of a pair of serially arranged filters. These filters are electronically switchable because they include an electronically controllable liquid crystal element. By electrically controlling the polarization of the liquid crystal element, the light which is transmitted through the filter may be controlled.
In this way, a given wavelength band may be transmitted through a given filter. In effect, the liquid crystal color filter may be tuned to a particular color. The liquid crystal color filters may be tuned to a pair of colors and through color combination, may produce a third color. Thus, a liquid crystal color filter may form a switchable shutter which can controllably produce red, green, and blue primary color bands or complementary color bands such as cyan, magenta and yellow.
Switchable shutters are commercially available. One commercially switchable shutter is the KALA switchable shutter produced by ColorLink, Inc., Boulder, Colo. 80301. A tunable color filter which may be used as one of the filters of a switchable shutter is described in U.S. Pat. No. 5,689,317 issued to Miller.
Switchable shutters may be used in color imaging systems to provide electronically switched color planes. In this way each image sensor can selectively be exposed to a particular color plane at one instance and then at sequential instances be exposed to successive color planes. Individual sensors are not necessary for each color plane but instead a single set of sensors may be used to sense each color plane in successive time intervals.
Thus, while switchable shutters and liquid crystal color filters offer important advantages, there is a continuing need for improved systems which may make these shutters and/or filters more advantageous in connection with digital imaging systems.
In accordance with one embodiment, a liquid crystal color filter includes a liquid crystal filter element that passes light in the visible spectrum. An infrared filter absorbs. infrared light and passes light in the visible spectrum. The infrared filter is incorporated into the liquid crystal filter element.